Apparatus for accurately controlling the production of semiconductor rods



y 26, 1964 J. GRABMAIER 3,134,694

APPARATUS FOR ACCURATELY CONTROLLING THE PRODUCTION OF SEMICONDUCTOR RODS Filed Aug 16. 1961 United States Patent 3,134,694 APPARATUS FOR ACCURATELY CONTROLLING THE PRODUCTION OF SEMICONDUCTOR RODS Josef Grabmaier; Munich, Germany, assignor to Siemens & Halske Aktiengesellschaft Berlin and Munich,

a corporation of Germany Filed Aug. 16, 1961, Ser. No. 131,753 Claims priority, application Germany Aug. 25, 1960 3 Claims. (Cl, 118-5) This invention relates to the production of semiconductor rods and is particularly concerned with a method of and a device for producing highly pure semiconductor rods by thermal decomposition of a gaseous compound of the semiconductor substance and separation of the semiconductor substance to be obtained upon a heated carrier made of the same material.

A rodshaped semiconductor may be produced, for example, by suitably disposing a rodshaped carrier body within a vessel, made for example of quartz, and heating such body, by direct passage of current therethrough, to the decomposing temperature of the gaseous semiconductor compound. The gaseous compound of the semiconductor substance which is to be decomposed, for example, a halogen compound of germanium or silicon, is preferably intermixed with a reducing gas, for example, hydrogen, such mixture being conducted into the reaction ves sel and thermallyvdecomposed therein, with formation of the free semiconductor substance, which is in highly pure and especially single crystal form separated (precipitated or deposited) upon the carrier body. The temperature of the carrier body is determined by the current flowing throughout the cross-sectional extent thereof, that is, by the current density. The separated semiconductor material effects radial growth or thickening of the semiconductor carrier rod, thus increasing the cross-sectional area thereof. Assuming that the current remains unchanged, the current density and therewith the temperature of the semiconductor rod will consequently decrease, thereby disturbing the growth and making the growth particularly of single crystal layers practically impossible.

In order to hold or adjust the temperature of the semiconductor rod during the growth thereof at a desired and especially at a constant value, the rod temperature was heretofore from time to time ascertained with the aid of a glow filament pyrometer, and was set to the desired value by manually regulating the heating current. Such regulation renders the radial growth of the semiconductor substance upon a rod-like carrier very cumbersome.

The object of the invention is to simplify the production process. According to the invention, an image of the glowing semiconductor rod, produced by an optical system, the diameter of which changes with the changing diameter of the growing semiconductor rod, is scanned by a movable photocell, and the contrast, bright-dark, appearing in the image plane at the border of the light beam bundle radiating from the semiconductor rod, is utilized for the control of a device which moves the photocell away from the light beam bundle and which simultaneously increases the current in the heating circuit to such an extent that the temperature of the semiconductor rod is held at a desired and especially at a constant value. v

The invention will now be described with reference to the accompanying drawing showing an example of an embodiment thereof.

Numeral 1 indicates in cross-sectional view a reaction vessel made of quartz, provided with a plane-parallel window 2, and containing a semiconductor rod, for example, a silicon rod 3 which is suitably disposed therein. The reaction gas mixture, which may contain doping matter if desired, for example, for the purpose of producing pnjunctions, is supplied through the inlet 4. Residual gases 1 are discharged through the outlet 5.

The silicon rod 3 is by current flowing therethrough in a suitable circuit (not shown) heated to a glow temperature of about 1100 C. The glowing rod represents a light source. The bundle of light rays 7 radiating from this light source extends through the optical system of a camera 8, indicated by the objective 6, with about 10 centimeter focal distance. The diameter of the light ray bundle is a measure for the diameter of the semiconductor rod 3. At the border of the light ray bundle appears in the image plane 9 a strong bright-dark contrast. A photocell 10 is disposed in the camera 8 in the image plane. The photocell 10 is after the heating of the carrier 3 to the desired temperature moved so close to the light ray bundle that flow of photo-current just ceases, at which time relay 11 is deenergized. The photocell 10 is fastened to a follower 12 which is movably disposed upon a spindle 13.

The reaction gas which is to be decomposed is now introduced into the reaction vessel and the liberated silicon is precipitated upon the heated carrier. The thickening of the semiconductor carrier rod results in an increase of the diameter of the radiated bundle of light rays. The light now falling upon the photocell 10 produces a photo-current which energizes the relay 11, the latter causing operative actuation of the motor 14. The motor rotates the spindle 13 to move the photocell 10 away from the image 9 of the light ray bundle until the light ceases to impact on the photocell, when the relay 11 deenergizes, causing the motor to stop.

The rotation of the spindle 13 during the motion of the photocell causes rotation of a gear disk 15 which meshes with the spindle. The disk 15, which need not necessarily be circular in shape, is in suitable manner coupled with means (not shown) for regulating the heating current for the semiconductor rod 3. For example, the rotation of the disk 15 can be utilized for actuating a variable resistor included in the heating circuit so as to reduce the resistance therein for the purpose of increasing the heating current to a value at which the semiconductor rod is maintained at the desired temperature. The rotation of the disk 15 can also be utilized for actuating a rotary transformer which is operative for increasing to a desired value the voltage of the heating current and therewith the current flowing through the semiconductor rod.

It is in the above described manner possible to effect an undisturbed growth of the semiconductor layer upon the carrier and to produce thereby highly pure single crystal semiconductor rods.

Since the rod thickens circumferentially practically uniformly, there will for symmetry purposes sufiice one photocell for carrying out the process. The temperature fluctuations of the semiconductor rod amount during the separation process, in the case of silicon separation in polycrystalline form, to about 30 C., and in the case of silicon separation in single crystal form, due to the resulting sharp border surfaces of the rod and consequently sharp imaging, to about :10 C. This accuracy has been found as being suflicient. In case it is desired to further reduce the temperature fluctuations during the separation process, a further photocell can be arranged in the image plane at the other border of the light ray bundle, and such further photocell can be employed for likewise controlling the operation of a device adapted to vary the current in the heating circuit.

Changes may be made within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

I claim:

1. A control device for use in the production of highly pure semiconductor rods by thermal decomposition of a gaseous compound of the semiconductor substance to be obtained and separation of such substance upon a glowing carrier body consisting of the identical substance and heated by the passage of an electrical current therethrough, comprising optical means for producing an image of the glowing carrier body, the diameter of which image changes with the changing diameter of the rod due to the thickening thereof by the deposit thereon of the separated substance, light-responsive means for measuring the bright-dark contrast appearing at the border of the image plane as a function of the rod diameter, and means coupled to said measuring means and responsive to operational changes thereof for controlling the current employed for heating said semiconductor rod.

2. In a device for producing highly pure semiconductor rods by thermal decomposition of a gaseous compound of the semiconductor substance to be obtained and separation of such substance upon a rodlike carrier consisting of the identical substance, which carrier is heated to glowing condition by direct passage of current therethrough, with said device maintaining substantially constant the temperature of the semiconductor rod which is being produced, throughout the production thereof, the combination of an optical system for producing an image of the glowing semiconductor rod, the diameter of said image changing with the changing diameter of the rod due to the thickening thereof by the deposit thereon of the separated substance, a bright-dark contrast appearing at the border of the image plane, photocell means disposed for response to said contrast, means for moving said photocell means with respect to said image plane, operating means for'controlling said photocell-moving means actuatable by the currentproduced by said photocell means in accordance with said contrast, and means governed by said operating means for controlling the current flowing through said semiconductor rod for the heating thereof to maintain the temperature of said rod substantially constant.

3. A device as defined in claim 2, wherein said means for moving the photocell comprises a follower movable along a rotatable spindle, said photocell being carried by such follower, and a gear disk operatively rotatable by said spindle operatively connected'to and comprising actuating means for said current controlling means.

References C ted in the file of this patent UNITED STATES PATENTS Great Britain Nov. 16, 1960 

1. A CONTROL DEVICE FOR THE USE IN THE PRODUCTION OF HIGHLY PURE SEMICONDUCTOR RODS BY THERMAL DECOMPOSITION OF A GASEOUS COMPOUND OF THE SEMICONDUCTOR SUBSTANCE TO BE OBTAINED AND SEPARATION OF SUCH SUBSTANCE UPON A GLOWING CARRIER BODY CONSISTING OF THE IDENTICAL SUBSTANCE AND HEATED BY THE PASSAGE OF AN ELECTRICAL CURRENT THERETHROUGH, COMPRISING OPTICAL MEANS FOR PRODUCING AN IMAGE OF THE GLOWING CARRIER BODY, THE DIAMETER OF WHICH IMAGE CHANGES WITH THE CHANGING DIAMETER OF THE ROD DUE TO THE THICKENING THEREOF BY THE DEPOSIT THEREON OF THE SEPARATED SUBSTANCE, LIGHT-RESPONSIVE MEANS FOR MEASURING THE BRIGHT-DARK CONTRAST APPEARING AT THE BORDER OF THE IMAGE PLANE AS A FUNCTION OF THE ROD DIAMETER, AND MEANS COUPLED TO SAID MEASURING MEANS AND RESPONSIVE TO OPERATIONAL CHANGES THEREOF FOR CONTROLLING THE CURRENT EMPLOYED FOR HEATING SAID SEMICONDUCTOR ROD. 